The present invention relates to a continuous preparation process of polytetrafluoroethylene wet powder.
Polytetrafluoroethylene (hereinafter referred to as "PTFE") dispersions are prepared by emulsion-polymerizing tetrafluoroethylene and are present in a form of aqueous dispersion in which fine particles of the emulsion-polymerized product having a particle size of 0.1 to 0.5 .mu.m are dispersed. It is difficult to obtain a PTFE fine powder directly from the aqueous dispersion, and also if possible, the resulting separated and dried powder is worse in handling. Therefore, hitherto a product (dry powder) of fine powder is prepared by applying a shear to the aqueous dispersion, flocculating the dispersed fine particles to produce a slurry, granulating the slurry with stirring to give a wet powder (these sequencial steps being, hereinafter, referred to as "agglomeration process"), and then drying the wet powder.
The agglomeration process is carried out by batch system or continuous system. In the batch system, the slurrying and granulation of the aqueous dispersion is carried out by only agitating in one agitation tank. This system, however, is inferior in workability and further the controlling of properties of the product such as an apparent density is difficult.
The continuous system is proposed in U.S. Pat. No. 3,046,263 and is a process by using a centrifugal pump and a capillary and a horizontal cylindrical agitation tank. According to this process, a PTFE aqueous dispersion is introduced to the capillary through the centrifugal pump to produce a slurry by shearing due to velocity gradient, and then the obtained slurry is granulated by the horizontal agitation tank to give a wet powder.
The prior continuous agglomeration system has, however, the following disadvantages. Namely, since the aqueous dispersion is supplied through the centrifugal pump to the capillary, when the centrifugal pump applies a shear, the fine particles partially flocculate to plug the capillary. Therefore, the capillary must be frequently changed due to plugging. Further, the shear velocity gradient becomes sometimes ununiform to give aggromelates having a wide particle size distribution. Amount of unagglomerated fine particles is not negligible. The horizontal agitation tank used for granulation usually has a both end supported shaft, and proper sealing is required at the end supports. In the agitation tank, the granules are taken off by introducing air into the agglomerates to rise to the surface. The floated granules, however, are broken by the agitation blades, and thus the produced powder becomes fine or deformed in shape. When an amount of fine powder is increased, an apparent density of the product powder becomes low, which makes handling property bad.
Accordingly, the production of PTFE powder is now carried out mainly by the batch system.